Any built structure transmits pressures to the ground which produce deformations in the ground that will emerge in the long teen, known as subsidence.
When at the base of two points of the same built structure the subsidences are different, the difference between the values measured is called a differential subsidence.
Usually differential subsidences of the ground at the base of built structures are not very significant and do not result in deformations of the structure that are such as to show cracks, collapses or malfunctions in general.
However, there are cases where the differential subsidences of the ground produce shifts of the overlying built structure that are such as to cause fractures in the structure, often non-negligible. These are cases where the ground is particularly deformable by the pressures transmitted by built structures, or cases where the built structures are constituted by fragile materials.
The method usually used to deal with this kind of problem, at the design phase of the built structure, or on existing built structures, involves a two-pronged analysis, of the foundation ground and the structure of the built structure.
The first analysis evaluates the nature and the texture of the ground and as a consequence makes it possible to calculate its resistance and deformability with respect to the loads of the built structure.
The second analysis examines the possible differential movements as a function of the type of structure planned in the design, or it reconstructs in detail the differential movements of the existing structure that have created the cracks present on the built structure, both in terms of time and in terms of geometry.
For the analysis of the ground, the designer can avail of traditional geotechnical tests, both on-site and in the laboratory, or of geophysical tests, which have recently been introduced on the market. The method derives substantially from traditional geotechnics and involves calculating the resistance and the deformability of the ground with respect to the pressures produced on the ground by the foundations of the built structure, starting from the geotechnical parameters gleaned from the texts.
The analysis of the built structure follows two different paths.
For buildings still to be built, the shifts are calculated according to the specifications of building science, optionally availing of digital models.
For existing buildings, the analysis of the built structure is much more comprehensive and complex than the one above, and uses measurement instruments associated with topography and with structural monitoring. Often leveling is carried out with precision instrumentation in order to verify which part of the built structure has subsided and the extent of the displacement. The topographic readings are then fleshed out by monitoring using crackmeters, inclinometers, strain gauges etc., the aim of which is to verify whether the subsidence is evolving and with what speed.
After completing the analysis on the foundation ground and on the structure of the built structure, the designer defines the most suitable method for resolving the differential subsidences.
There are various systems for preventing or resolving differential subsidences. They are divided into systems that act on the structure and systems that treat the ground.
The first systems seek to modify how the pressures of the built structure are transferred to the ground through works adapted to widen the base of the foundation or to deepen it in the ground until it meets strata that are more substantial therefore resistant. The methods in question are generally applied to the entire structure: these systems include, for example, micropiles and underpinning.
The second systems seek to improve the characteristics of resistance and deformability of the ground through actions aimed at densifying the mass or at introducing materials or mixtures into it that physically or chemically modify the characteristics of the natural ground. These methods can be limited to some portions of the built structure, where the ground has poorer characteristics. This category includes, among others, injections of cement and/or of synthetic resins.
Among the various known methods for treating the ground to resolve differential subsidences by way of injections, is for example the method described by EP0851064, which entails increasing the load-bearing capacity of foundation grounds for buildings by way of injecting a substance that expands following a chemical reaction. The method disclosed includes verification of the effectiveness of the measure, by way of using laser receivers fixed to some points of the structure overlying the injected volume which, connected to an emitter, report the vertical shifts of the built structure following the expansion of the substance in the ground.
There are other methods that involve the injection of mixtures of differing nature. One of these is the Soilfrac technology developed by the Keller Grundbau company, which entails the use of mixtures of cements and expanding cements. The method creates fractures in the ground in multiple steps, using the mixture which is injected by way of a pump that develops medium-to-high pressures. In this case too, shifts of the building or of the ground overlying the injection are monitored by way of levelometer systems that make it possible to observe the relative movement of some points of the built structure with respect to others, using the principle of communicating vessels.
The aim of the monitoring systems described that are availed of by the known methods is to indirectly evaluate the effectiveness of the intervention, i.e. they detect consolidation occurring in the ground through observation of the movement of the structure overlying the treated point, or of the surface of the ground.
The methods described, although indirect, are very widely used because they are immediate and low cost and they do not entail invasive measures in the ground.
Both of the systems described have limitations, however, which are due to the fact that the rise of the built structure or of the surface of the ground overlying the injection, although necessary, is not always sufficient to ensure the adequate improvement of the entire volume of ground affected by the loads of the structure.
It can in fact happen that the building or the surface of the ground registers a rise owing to overpressures of water contained in the gaps of the ground, but this is not sufficient to ensure adequate values of load-bearing capacity in the long term. It can further transpire that the rise of the built structure or of the surface of the ground is due to the pressures that the injected mixture exerts at a certain depth in the ground, but this is not an indicator of complete improvement of the volume of ground present throughout the vertical underlying the point being monitored.
The vertical movement of the building as a result of the injections depends greatly on the weight and on the rigidity of the structure. Smaller buildings with mostly isostatic constraints are affected locally by pressures in the ground, while larger buildings with more complex and rigid structures are less likely to rise, since larger portions of the structure are affected. It is especially with this latter type of building that the criterion of effectiveness means it is not possible to evaluate the homogeneity of the treatment of the ground with precision. In fact it can happen that an entire portion of built structure rises uniformly, even if in reality the consolidation obtained with the injections does not affect the entire volume of ground but only a reduced portion of it.
In order to overcome this indeterminacy dictated by the imprecision of the indirect measurement method, the measures carried out on rigid structures, but also on other structures, are generally overdimensioned, i.e. they follow very dense injection geometries that rely on the overlapping of the effects since they are not perfectly controllable.
In order to obtain the improvement of the mechanical and hydraulic characteristics of all of the treated volume of ground, the methods that avail of indirect evaluation of effectiveness by observing the movement of the structure overlying the treated point or of the surface of the ground force the designers to follow non-optimal distribution criteria of the injections, with consequent increase of the cost and time for carrying out the work.
Furthermore, even after carrying out a superabundant number of injections with respect to the number theoretically necessary, and even after injecting an amount of cement and/or synthetic mixture that produces a rise, possibly major, of the overlying built structure, it is not certain that the improvement of the mechanical and/or hydraulic characteristics of the ground will be homogeneous as described earlier.
In fact, the rise could be determined by a temporary increase in the pressure of the water contained in the gaps of the ground, or it could be determined, in areas farther away from the injection point, by the rigidity of the structure and it may therefore not be a good indicator of effectiveness of the injections.
Furthermore, the detection of the rise during the injection step is done exactly, generally with a laser level that measures the vertical displacement of a point of the structure. Such point can be above or below a crack, resulting in a signal that is sometimes deceptive.
The volume of influence of the injection of cement and/or synthetic mixture strictly depends, in addition to on the type of mixture, which may or may not be expanding, on the amount of mixture dispensed, on the physical and mechanical characteristics of the ground, and on the injection parameters such as the pressure and the temperature.
Other methods of consolidating ground are known, although they differ from the ones described here; they use systems of controlling the injections of chemical substances into the ground by way of geoelectrical surveys. An example of this is the method described in European patent EP1914350 which involves the use of 3D tomography of the electrical resistivity of the ground in comparative form, i.e. carried out both in the ground that is not affected by subsidence phenomena and in subsided portions. In this case, the aim of the electrical tomographic measurement in the natural ground that has not subsided has the aim of defining the electrical resistivity values to take as a reference for the operation to consolidate the ground, while the tomography carried out in the ground affected by subsidence has the aim first of all of defining a starting value and subsequently of checking the evolution of the resistivity values during the injections, which will need to lean towards the values measured in the area that has not subsided.
Such system however presents clear limitations which can briefly be summed up in the following points. The area on which it is decided to monitor the reference electrical resistivity, presumed to be an area not affected by subsidence, can in reality present problems of poor load-bearing capacity and therefore it can lead the operator to set target electrical resistivity values that are incorrect. In effect there can be portions of the building that are not hit by instability which in reality stand on areas of ground that do not have sufficient load-bearing capacity, but which remain integral because they are very strong and rigid.
The case can also arise where the entire building is hit uniformly by instability and therefore it is not possible to define the reference electrical resistivity value.
In the same way, it is impossible to compare electrical resistivity values if the portion of building that has subsided stands on grounds of a different nature from those in the portion that has not subsided, or where the geometry of the foundations is evidently different between the various portions.
There is further a frequent type of case where the subsidence is linked to phenomena of drying of clayey ground. In this case the reference tomography, i.e. the tomography carried out on the area that has not subsided, will from the beginning have lower resistivity values than those of the subsided portion. Therefore, any injection operation carried out in the subsided area which improves the resistivity of the ground cannot make the measured values lean towards the reference values.
In all the cases examined, however, the electrical resistivity values measured do not make it possible to evaluate the efficacy of the operation because they are limited to describing a parameter associated with the electrical properties of the ground, which is very different from the mechanical parameters used to evaluate the state of consolidation.
The method described further does not use a system for controlling shifts of the building and therefore it does not ensure the required safety during the injection step. It can happen in fact that the treatment of the ground by way of injection, aimed only at varying the electrical resistivity, can produce shifts of the overlying built structure which are such as to induce angular distortions in the structure which are greater than the tolerances allowed.
Angular distortions are defined as the ratio between the differential vertical displacement between two points of the same built structure (differential subsidence or differential lifting) and their minimum distance. The skilled person is always capable of determining the admissible tolerances with the help, for example, of some tables that contain the admissible and limit values for the angular distortions as a function of the type of building. By way of non-exhaustive example, below are the most significant:
Limit Values for Angular Distortions According to Bjerrum (1963)
Category of potential damagetanβLimit beyond which problems can arise1/750in machinery sensitive to subsidencesDanger limit for space frame structures1/600Safety limit for buildings in which no1/500cracking is admittedLimit beyond which the first cracks can1/300appear in dividing walls and difficultyin use of bridge cranesLimit beyond which inclinations in tall1/250buildings can be visibleConsiderable cracks in dividing walls1/150and supporting brick wallsSafety limit for supporting brick walls1/150with h/L < 1/4Limit beyond which structural damages31/12/49is to be expected in buildings
Admissible Angular Distortions According to Sowers (1962)
Type of structuretanβMultistory load-bearing walls0.0005 ÷ 0.001Single-story load-bearing walls0.001 ÷ 0.02Plaster cracks0.001Reinforced concrete frames0.0025 ÷ 0.004Walls of reinforced concrete frame structures0.003Steel frames0.002Simple steel structures0.005
The admissible angular distortion values for the built structure under study are defined at the design stage.
Application of a method of consolidating the ground that does not make use of a system of monitoring the built structures overlying the treated volumes of ground is therefore excessively risky and certainly lacking control.
Another conventional method that uses 3D tomography of electrical resistivity in consolidation of the ground is the method described in EP2543769 which entails consolidation of the ground and the simultaneous sequential use of electrical tomography. The aim of the geophysical survey in this case is to quantify the value of electrical resistivity in order to provide the operator with indications on the criterion for interrupting the injection. The method in fact indicates as a criterion for stopping the injection the moment when, between two successive injections, the variation in electrical resistivity acquired by tomography is lower than 5%.
In this case too the method exhibits limitations. Consider for example the case where the ground has subsided owing to drying and is therefore in conditions of very low humidity. The value of resistivity measured in the initial step, as mentioned, is very high and therefore it can happen that the subsequent value measured after the first injection only will have increased by a percentage of less than 5% with respect to the initial value measured. In this case the method requires stopping the injection, even if sufficient consolidation of the ground has not been achieved.
Of further note, also in this case, is the absence of monitoring of the building, which does not give adequate safety assurances during the injection step against the development of admissible angular distortions on the structure.
The monitoring of the variation of resistivity in a volume of ground entails changes that are different from point to point. There are in fact points where the variation is marked and others where it has little significance. The method described does not specify which are the volumes to consider in applying the efficacy criterion or whether the reference value is the average. Also in this case, the fact remains that the method is based exclusively on an evaluation linked to the electrical properties of the ground which represent an indirect and imprecise measurement of the mechanical characteristics of the ground.
Finally, while the methods of ground consolidation that avail only of the system of measuring the electrical resistivity on the one hand make it possible to identify at least approximately the volume of ground that is affected by the effects of the injections, on the other hand they cannot evaluate the intensity thereof that is necessary to ensure the improvement of the mechanical characteristics.
In fact, it can happen that even defining a spatial distribution of the injections that makes it possible to obtain a significant variation in electrical resistivity of the volume treated, with the consolidation methods described it is not possible to define the correct amount of cement and/or synthetic mixture to be used for the individual injections in order to obtain the desired consolidation and prevent excessive angular distortions on the built structures on the surface.